The invention describes a method for developing three-dimensional refractive index structures using optical irradiation and, more particularly, a method using photo-patterning technology to develop three-dimensional refractive index structures in layered, stacked, photosensitive thin film materials.
Photo-lithographic patterning typically focuses on the formation of two-dimensional patterns in a photoresist material, involving the use of multiple chemical and mechanical processing steps and material deposition steps. In contrast, photosensitive materials are those materials into which a stable refractive index change can be photo-imprinted, such as by using a single-step, direct writing approach. This provides an opportunity over more conventional photolithography for rapid, agile manufacture of integrated photonic device structures useful for optical signal manipulation and remote environmental sensing. Photosensitive Bragg gratings, in fact, are the basis for a wide range of photonic devices that are used in optical telecommunication applications. Such gratings are photo-imprinted into the core of an optical fiber to provide such functions as spectral filtering and dispersion compensation.
In photolithography, a positive or negative image of the desired configuration is first introduced into a photoresist material by exposing it to patterned radiation that induces a chemical change in the exposed portions of a resist material. This chemical change is then exploited to develop a pattern in the material, which is then transferred into the substrate underlying the resist. Photo-patterning is used to alter the chemical resistance of photoresist material to chemical attack. The photoresist material is not retained in the finished product. The photo-patterned photoresist provides an opportunity to selectively deposit materials at precise locations over the surface of the piece or to selectively etch certain locations. Thus, refractive index structures are usually formed using dissimilar material types (such as glass versus crystalline or insulator versus semiconductor) or material compositions (varied dopant identities or concentrations) whose spatial distribution is formed via the photo-patterning of photoresist layers. Because of this use of multiple materials, multiple photo-patterning steps and chemical processing of the piece are generally required. These steps also require precise registry of the mask images used in consecutive photo-patterning steps.
Other methods for the formation of refractive index structures include electron and ion beam etching (including reactive ion etching). In these cases, spatial patterning of the refractive index is dictated by selective exposure (etching) of the parent material with a particle beam. This is followed by further thermal or chemical processing and/or deposition of dissimilar materials to provide a finished product. These energetic particle approaches require vacuum chamber technology and can be expensive. With control of the etching conditions, it is possible to build in some control of the structure cross section with depth into the material but the types of structure possible are limited and, again, the overall process is multi-step in nature, yielding a heterogeneous material structure. Mechanical machining might also be used but the difficulties in the formation of complex shapes at a small scale tend to make this approach expensive and not amenable to large volume production.
In direct writing of index modulations using a photosensitive materials approach, the processing techniques and conditions are typically more benign and can be applied to varied bulk material geometries, e.g. fiber and thin film. Common to both fiber and thin film implementations of photosensitivity, however, the photo-imprinted refractive index structure is typically only defined in two dimensions within the material. In most cases, the refractive index profile does not vary with the depth into the photosensitive material. The production of three-dimensionally defined refractive index structures would greatly increase the versatility and applicability of photosensitive materials.
In conventional, homogeneous photosensitive materials, the photosensitive change in refractive index is initiated via a one-photon absorption process into an absorption band of the material located at some characteristic wavelength, xcex. The wavelength of the light used to imprint the refractive index change, referred to as the writing wavelength, is tuned to access the wavelength xcex characteristic of the material. Hereinafter, writing an index structure means that the refractive index of some volume of the material is changed through exposure to optical radiation, thereby forming a structure (a volume of the material within the material with a refractive index different than the original material). However, formation of subsurface structures (that is, structures formed entirely in the interior of the material) in a homogeneous material is not possible using a single writing wavelength because the optical radiation must first traverse the material from the surface to the desired target volume element. Because the entire material volume is photosensitive, that intermediate material will also undergo a refractive index change.
Three-dimensional structures in a homogeneous material have been formed using optical illumination systems (see U.S. patent application Ser. No. 09/492,956, filed on Jan. 27, 2000, now U.S. Pat. No. 6,368,775 incorporated herein by reference) and also using composite materials (see U.S. patent application Ser. No. 09/788,052, filed on Feb. 16, 2001; incorporated herein by reference). Mustacich (U.S. Pat. No. 5,932,397, issued on Aug. 3,1999) describes formation of three-dimensional gradients in a homogeneous material through control of the distribution of wavelengths used in illuminating the material. Yoshimura et al. (U.S. Pat. No. 6,081,632, issued on Jun. 27, 2000) describe forming optical waveguide systems in a homogeneous material to produce a refractive index distribution while inducing self-focusing and insolubilizing the photosensitive material.
Useful would be a method for forming three-dimensional structures in a photosensitive material where these structures are formed with desired refractive index three-dimensional profiles.